We have continued to focus our attention on the interaction of Campylobacter jejuni with human epithelial cells particularly with respect to altered protein synthesis that occurs during the binding and internalization of C. jejuni. Antisera prepared against organisms grown in the presence of epithelial cells are capable of inhibiting translocation and have been used to screen recombinant expression libraries of C. jejuni genomic DNA. Several clones have been identified that produce proteins that are preferentially recognized by antisera produced against bacteria cultured with epithelial cells. One clone has been completely sequenced and appears to encode the homolog of the bacterial DNA-binding protein HU. Additional studies have characterized in vitro phenotypic passage variants of C. jejuni. A high passage variant of a clinical isolate termed M96 that is defective for internalization within epithelial cells has also been obtained. However, this variant is fully competent at translocating across monolayers of polarized epithelial cells. At the molecular level, the strain differs only from the parent in terms of the structure of lipopolysaccharide (LPS). This finding may indicate a specific role for LPS in internalization and further suggests that translocation across polarized cells occurs by a different mechanism than that associated with internalization or invasion of conventionally cultured cells.